Circuit arrangement for the operation of photo-electric cells



Dec. 10, 1929. "A, KAROLUS 1,739,005

CIRCUIT ARRANGEMENT FOR THE OPERATION OF PHOTO ELECTRIC CELLS Filed Oct.10, 1927 INVENTOR AUGUST KAROLUS.

TORNEY Patented Dec. 10, 1929' UNITED. STATES PATENT OFFICE AUGUSTKAROLUS, or 'LFJPSIG, GERMANY, ASSIGNOR r0 RADIO CORPORATION or AMERICA,OI NEW YORK, N. Y., A CORPORATION OF DELAWARE CIRCUIT ARRANGEMENT FORTHEOPERATION OF PHOTO-ELECTRIC CELLS Application filed October 10, 1927,Serial Ito. 225,065, and in Germany September 29, 1926.

An application for this invention filed in Germany September 29, 1926.

The invention relates to the use of photoelectric cells of all kinds,whether of the high vacuum or gas filled kind, for the conversion oflight variations into electrical actions, for instance, for the purposeof picture transmis sion and related arts. According to former rentsource and ohmic resistance, and, due to the fluctuations of the cellcurrents produced by varying intensities of light reaching the cell andcausing varying Voltage drops across the resistance, varying potentialswere applied on thegrid of an amplifier tube havingits input circuitconnected across the resist ance member. Further above in the amplifiercascade arrangement there was generally introduced an alternatingcurrent potential of convenient frequency in the shape of a carrierwave, in order that the very wide frequencybandof the current variationsresulting from the picture exploration may conveniently andundistortedly be further amplified. The picture-point frequencies appearin this scheme as modulations of the carrier wave introduced into thecascade arran ement.

ow, in an arrangement ofthis kind, there resulted difliculties ofmanipulation of the amplifiers, and these were due to the fact that thesensitivity of present-day photo-electric cells is not quite constant.As a result, the value of the direct current grid potentialcorresponding to a definite illumination of the photo-electric cell wasfound to undergo changes at the amplifier tube whose grid di rectcurrent potential is superposed upon the carrier fre uenoy, and this inturn results in alterations 0th of the amplitude as well as of thedegree of modulation of the carrier oscillations. When these were usedfor con-' trolling a radio transmitter, then similar fluctuations inboth directions were caused also in the complete modulation of thetransmitter, with the result that difiiculties usually arose in picturetransmission. The present invention obviates this inconvenience, byusing an alternating current potential whose frequency at the same timegoverns the carrier frequency to be further amplified is used as asuction or anode potential at the photo-electric cell for the electronsreleased by the light. A pure alternating current potential must beused, or else the alternating current potential is superposed upon aconstant direct current potential. Also an intermittent direct currentpotential obtained by the rectification of alternating current may beemployed. To insure the desired effect either one of the said schemes ispracticable.

ing current potential is usedas the suction By virtue of the fact thatan alternator anode potential of the electrons, the current of thiscarrier frequency in the absence of light will always be completelycontrolled, since the photo-electric cell is then impermeable. It isthen also possible to apply a perfectly linear amplification even to thefirst amplifiertube, upon the grid of which the photo-electric cellcurrents are brought to act, and manipulation of the arrangementbecomesfree from such diificulties as usually arise in connection withthe use of a pure direct current potential to act as the suction oranode potential of the photo-electric cell. The value of the alternatingcurrent suction potential, or the sumtotal of direct currentandalternating current potential, may be so chosen in practice thatingas-filled cells the glow-discharge potential is nearly reached or evenexceeded at the crest values of the alternating current potential. Inthis manner, a supplementary positive ionization is obtained, and thusalarger current effect in dependence upon the light. It is possible into the maximum picture-point frequency which is to be transmitted, inother Words, it should be correspondingly higher than the latter. 4

In order that in a method as hereinbefore disclosed the harmful actionof electrode capacity of the photo-electric cell may be precluded,any'alternating current as may flow across the cell by capacity actionmust be compensated. For this object any suitable sort of compensationscheme may be used, for instance, a bridge arrangement of the kind shownin the drawing which forms a part of this specification.

Referring to thedrawing, 1 is the photoelectric cell which conjointlywith the variable compensating condenser 3 and the alternating currentresistances 4 and 5 represents the arms of the bridge 2. At theterminals 6 the alternating current potential of convenient frequencyand amplitude is introduced, while a suitable direct current potentialmay in addition be connected in series with the cell 1. When thealternating current resistances 4 and 5 are equal, then the adjustablecapacity 3 must be made equal to the electrode capacity of cell 1.Hence, in this arrangement any flow of current due to capacity acrossthe cell is completely neutralized, since the capacity 3 provides thatno potential difference will result between grid and filament ofamplifier tube 7. The alternating current potentials there arising aremerely due to the electronic and ionic con duction current when cell 1is illuminated, and

7 they fluctuate in their amplitude in proportion to irradiation.Further amplification may then be effected conveniently by way of atransformer coupling 8 through which the alternating current potentialsare fed to additional tubes.

I claim as my invention:

1. An arrangement for modulating a carrier frequency in' accordance withvariations in light intensity which includes, a bridge- I structure, aphoto cell connected to form one leg of saidbridge structure, a capacityforming a leg of said bridge structure adjacent saldjphoto cell, saidcapacity being of the order of magnitude of the capacity of said cellelectrodes for conditions of minimum illumination on said photo cell,and a plurality of resistors connected to form the other two legs ofsaid bridge, means for applying a carrier frequency potential atopposite ends of the diagonal of said bridge structure between the pointof connection of said cell and capacity at one end and the point ofcon-- nection of the said resistors at the other end, so .as to provideby said carrier frequency potential a source of anode potential for saidcell, and means connected across the second diagonal of said bridge foramplifying said carrier fr uency when modulated by hght changes a ectingsaidphoto cell.

2. An arrangement for modulating a carrier frequency in accordance withvariations in light intensity which includes, a bridge structure, aphoto cell connected to form one leg of said bridge structure, avariable ca-' pacity forming a leg of said bridge structure adjacentsaid photo cell, said variable capacity being adjusted to be of theorder of magnitude of the capacity between said photo cell electrodesfor conditions of minimum illumination on said photo cell and forming abalance for said bridge. means for applying a carrier frequencypotential at opposite ends of the diagonal of said bridge structurebetween said cell and capacity so as to provide by said carrierfrequency potential a source of anode potential for said cell, and meansconnected across the second diagonal of said bridge for amplifying saidcarrier frequency when modulated by light changes affecting said photocell.

3. An arrangement for modulating carrier frequencies in accordance withvarying fluctuations in light intensities which includes, a bridgestructure, a photo cell forming one leg of said bridge structure, avariable capacity forming a leg of said bridge structure adjacent saidphoto cell, and resistances of equal value forming the other two legs ofsaid bridge structure, said variable capacity being of such value thatthe capacity between the photo cell electrodes for minimum illuminationon said photo cell is compensated and an unbalanced condition in saidbridge structure is obtained only through variations in the conductioncurrent through said photo cell produced by varying light fallingthereon, means for impressing a carrier frequency potential upon saidcell in a manner to utilize said carrier frequency as a source of anodepotential for said cell by connecting the said carrier frequencypotential across one diagonal of said bridge between'said cell and saidcapacity at one end and between said resistances at the other end, andan amplifying means connected across the opposite diagonal of saidbridge for amplifying said carrier frequency when modulated by varyinglight values changing the conduction currents through said photo cell.

4. An arrangement for modulating a carrier frequency in accordance withvarying intensities of light and shade, which includes, a bridgestructure having a photo cell arranged to form one leg thereof, acapacity forming. an adjacent leg thereof, and a plurality of impedancemembers forming the other two legs of said bridge, said photo cell beingsubjected to varying intensities of light and shade for producingtherein a varying members at the other end a carrier frequency potentialof a Value such that the glow discharge potential of the cell issubstantially reached when the carrier frequency alte1' hating currentpotential has its crest value across the said diagonal, and meansconnected across the second diagonal of said bridge for amplifying saidcarrier frequency po tential as modulated in accordance With variationsproduced in said photo cell, the variations in light and shade reachingthe same.

AUGUST, KAROLUS.

